Method for preparing pipe for butt welding

ABSTRACT

A steel pipe is welded to a threaded connector that has an end with a desired nominal inner diameter. Initially, the inner diameter of the steel pipe may be out of a tolerance range of the nominal inner diameter of the connector. A swage is forced over an end of the pipe, reducing an outer diameter and an initial inner diameter of the end of the pipe. The swage is dimensioned to provide the end of the pipe with an inner diameter that is less than the nominal inner diameter of end of the connector. A die is then forced into the end of the pipe, the die having an outer diameter sized to increase the inner diameter of the end of the first tubular member to the nominal inner diameter of the end of the second tubular member. A weld groove is machined on the end of the pipe and the ends are abutted and welded at the weld grooves.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to provisional application No.60/366,290 filed Mar. 21, 2002.

FIELD OF THE INVENTION

[0002] This invention relates in general to preparing steel pipe forbutt welding, particularly to a threaded connector.

BACKGROUND OF THE INVENTION

[0003] Subsea well production risers typically are made up of sectionsof pipe joined together with threaded connectors. In one type ofproduction riser, the threaded connectors are formed separately andwelded to the ends of pipe. These risers are subject to large bendingforces due to wave motion, wind and currents. As a result, the risersare subject to severe fatigue and tensile loadings from the bending andweight of the riser string. Generally, the weakest point is at the weld.

[0004] One of the problems associated with welding the threadedconnectors to the pipe is that often the pipe will not be preciselydimensioned at the weld. The pipe may be out of round or have an innerdiameter that is too small or too large. The connector is a machinedpiece, thus its weld end is very accurate with respect to roundness andinner diameter. A typical tolerance might be plus or minus 0.010″,although it could be smaller. It is very expensive to specify such hightolerances for pipe from a steel mill. For conventional API pipe, theinner diameter is governed by tolerances on the outer diameter and wallthickness, and is held to approximately plus or minus 2%. In specialcases, mills have held inner diameter tolerances to about plus or minus0.062″ at a significant increase in cost.

[0005] Consequently, a mismatch at the inner diameters of the pipe andconnector is likely to occur and result in inferior welds. This mismatchmay lead to an early failure of the weld due to stress concentrations atthe point of mismatch. Any mismatch on the exterior can be groundsmooth, but a grinding operation in the inner diameter after weldingwould be expensive and time consuming.

[0006] In the prior art of machining threads directly on pipe ends, itis known to perform sizing operations on pipe ends prior to machiningthe threads. To applicant's knowledge, however, no one has performedsizing operations on a pipe end to achieve a tight tolerance innerdiameter, then butt welding the pipe end to a threaded connector.

SUMMARY OF THE INVENTION

[0007] In this invention, the pipe is conventional and held only to thetypical mill tolerances. The connector, being machined, will have an endthat is within tolerances for roundness and inner diameter. To preparethe pipe, first an end portion is swaged to reduce an initial innerdiameter of the end portion. Then, the inner diameter of the end portionis expanded to the desired nominal inner diameter. A weld groove ismachined on the end portion.

[0008] Preferably, the end portion is heat treated after the expansionstep to reduce residual stresses due to cold working. The swaging stepis preferably performed by forcing a tubular swage over an outerdiameter of the first tubular member. The expansion step is preferablyperformed by forcing a die into the inner diameter of the end portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a sectional view of a pipe end and connector weldedtogether in accordance with this invention.

[0010]FIG. 2 is a schematic view illustrating the pipe end of FIG. 1undergoing a preparatory step of swaging.

[0011]FIG. 3 is a schematic view of the pipe end of FIG. 2, afterswaging, and shown undergoing a punching operation to enlarge thediameter of the swaged portion.

[0012]FIG. 4 is a schematic view of the pipe end of FIG. 2 after beingenlarged by the punch operation of FIG. 3 and shown being heat-treated.

[0013]FIG. 5 is a schematic view of the pipe end of FIG. 2, illustratinga weld groove being machined.

[0014]FIG. 6 is a schematic of the pipe end of FIG. 2, shown abuttedwith the connector of FIG. 1 for welding.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to FIG. 1, a threaded connector 11 is shown. Connector11 is a tubular member preferably formed of steel and having a bore 13through it. Connector 11 has threads 15 on one end that may be a varietyof types. The opposite end 16 is joined to a steel pipe 17 by a weld 19.Typically, bore 13 will be formed with an inner diameter having atolerance of plus or minus 0.010″, although this could be smaller.

[0016] Referring to FIG. 1, connector 11 and pipe 17 are used to formsections of a production riser for subsea well production. Referring toFIG. 2, a few inches of the end of pipe 17, such as 4″, are sized withindesired tolerances to match the nominal inner diameter and roundness ofthe weld end 16 of connector 11. This sizing is performed in a coldworking process by first swaging the end of pipe 17 with a swage die 23.Swage die 23 may be of any conventional type for swaging a tubularmember to a smaller diameter. Swage die 23 may move over pipe 17 orvice-versa, or both members may move toward each other. Swage die 23 hasa bore with a tapered entry 25 and an inner diameter 27 that is smallerthan the nominal outer diameter of weld end 16 of connector 11.

[0017] Inner diameter 27 is sized so that it will always be smaller thanthe outer diameter of pipe 17, even if the outer diameter is at itsminimum tolerance. Consequently, swage die 23 will create a reducedinner diameter section 29 and a reduced outer diameter section 31 as itis brought over the end of pipe 17 illustrated in FIG. 3. The reduceddiameter sections 29, 31 in FIG. 3 are shown greatly exaggerated forillustration purposes. The reduction in inner diameter 29 as a result ofthis swaging operation is from 1% to 5%, preferably about 3%. Thedimension of reduced inner diameter section 29 will be less than thenominal inner diameter of connector end 16.

[0018] Next, reduced inner diameter section 29 undergoes a cold workingenlargement with a die or punch 33 illustrated in FIG. 3. Punch 33 hasan outer diameter 35 that is cylindrical and has a tapered section 37 onits forward end. The outer diameter 35 of punch 33 is the same as thenominal inner diameter of connector end 16 within a tolerance ofpreferably plus or minus 0.010″ in the preferred embodiment, although itoptionally could have a tolerance as much as 0.020″. When punch 33enters reduced inner diameter section 29, it will permanently deform theinner diameter section 29 to the nominal diameter of connector end 16.During the enlarging process, either punch 33 can be pushed into pipe17, or vice versa, or both may be moved toward each other.

[0019]FIG. 4 shows nominal inner diameter section 39 after undergoingthe enlargement procedure illustrated in FIG. 3. Inner diameter section39 will now be within acceptable tolerances for roundness as well asinner diameter, these tolerances preferably being plus or minus 0.010″and as much as 0.020″. The reduced outer diameter section 31 alsoenlarges while punch 33 enters inner diameter section 29 (FIG. 2). Outerdiameter section 41 may not necessarily be the same as the outerdiameter of connector end 16 (within plus or minus 0.010″), because theinitial wall thickness of pipe 17 may have differed from the wallthickness of connector end 16 by a tolerance greater than 0.010″. Anyvariations on the exterior may be ground off smooth after welding,however.

[0020] As illustrated in FIG. 4, preferably the end of pipe 17 is heattreated after nominal inner diameter section 39 has been achieved toremove residual stresses caused by the cold working of the processesshown in FIGS. 2 and 3. The amount of heat is not high enough to reachthe lowest transformation temperature of the steel, rather it ismaintained at a much lower level, such as 800° F. The heating may beapplied by a variety of means, but is preferably handled by an inductioncoil 43 placed over inner diameter section 39 and outer diameter section41. Preferably the cooling is by air.

[0021] Referring to FIG. 5, after heat treating, a weld groove 45 isformed on pipe 17. Weld groove 45 may be a variety of shapes and isgenerally a concave recess or bevel. A cutting tool 47 is used to formgroove 45. A follower 49 connected with cutting tool 47 engages thenominal inner diameter section 39 during the machining process to assurethat weld groove 45 is concentric with nominal inner diameter section39. Follower 49 controls the position of cutting tool 47. Machine toolshaving followers such as follower 49 and a cutting tool 47 areconventional.

[0022] Then, connector 11 is abutted against pipe 17 as illustrated inFIG. 6. The inner diameter 13 of connector end 16 will have beenpreviously machined to be at the nominal dimension within tolerances.Because of the swaging and enlarging processes of FIGS. 2 and 3, nominalinner diameter section 39 will be at the same dimension withintolerances. Weld groove 51 on connector 11 will mate with weld groove45. A fixture 53 of conventional design has gripping members 55 thatgrip bores 13 and 21 to align and hold connector 11 in abutment withpipe 17. The welding may be of a variety of types such as TIG, submergedarc or MIG. After the welding is completed, preferably the outerdiameter 41 of pipe end 17 is ground flush with the outer diameter ofconnector end 16.

[0023] The invention has significant advantages. The method results in abetter weld because the inner diameters will match within tolerances.Conventional mill pipe may be employed without increasing tolerances.The steps of swaging, enlarging and heat treating can be performed withconventional equipment.

[0024] While the sizing and welding operations described above are inconnection with securing a connector to a pipe, this could also beperformed when connecting two pipes together where the pipes will beundergoing severe bending and fatigue stresses. In the example above,the outer diameter of pipe 17 is 10¾″ but diameters could range fromabout 9⅝″ to 16″ for offshore production risers of this nature.

[0025] While the invention has been shown in only one of its forms, itshould be apparent to those skilled in the art that it is not so limitedbut is susceptible to various changes without departing from the scopeof the invention.

I claim:
 1. A method for preparing a first tubular member for butt-welding to second tubular member, comprising: (a) swaging an end portion of the first tubular member to reduce an initial inner diameter of the end portion; (b) after the inner diameter has been reduced, expanding the inner diameter of the end portion to a desired nominal inner diameter; and (c) machining a weld groove on the end portion.
 2. The method according to claim 1, wherein step (a) is performed by forcing a tubular swage over an outer diameter of the first tubular member.
 3. The method according to claim 1, wherein step (b) is performed by forcing a die into the inner diameter of the end portion.
 4. The method according to claim 1, wherein step (c) is performed after step (b).
 5. The method according to claim 1, further comprising heat treating the end portion after step (b) to reduce internal stresses.
 6. The method according to claim 1, further comprising heat treating the end portion after step (b) and before step (c) at a temperature below a lowest transformation temperature of the material of the first tubular member.
 7. The method according to claim 1, wherein step (b) results in a reduction of the initial inner diameter in the range from 1% to 5%.
 8. The method according to claim 1, wherein steps (a) and (b) are performed by cold working processes.
 9. The method according to claim 1, wherein the nominal inner diameter of step (b) is within a tolerance of plus or minus 0.020 inch.
 10. The method according to claim 1, wherein the nominal inner diameter of step (b) is within a tolerance of plus or minus 0.010 inch.
 11. A method for welding a first tubular member to a second tubular member that has an end with a desired nominal inner diameter and a weld groove, comprising: (a) swaging an end of the first tubular member to reduce an initial inner diameter of the end of the first tubular member to less than the nominal inner diameter of end of the second tubular member; then (b) forcing a die into the end of the first tubular member to increase the inner diameter of the end of the first tubular member to the nominal inner diameter of the end of the second tubular member within a tolerance of plus or minus 0.020 inch; then (c) machining a weld groove on the end of the first tubular member; then (d) abutting the end of the first tubular member to the end of the second tubular member and welding the end portions together at the weld grooves.
 12. The method according to claim 10, wherein step (a) is performed by forcing a tubular swage over an outer diameter of the first tubular member.
 13. The method according to claim 10, further comprising heat treating the end portion after step (b) and before step (c) at a temperature below a lowest transformation temperature of the material of the first tubular member.
 14. The method according to claim 10, wherein step (b) results in a reduction of the initial inner diameter in the range from 1% to 5%.
 15. The method according to claim 10, wherein steps (a) and (b) are performed by cold working processes.
 16. A method for welding a steel pipe to a threaded connector that has an end with a desired nominal inner diameter and a weld groove, comprising: (a) forcing a swage over an end of the pipe, reducing an outer diameter and an initial inner diameter of the end of the pipe, the swage being dimensioned to provide the end of the pipe with an inner diameter that is less than the nominal inner diameter of end of the connector; then (b) forcing a die into the end of the pipe, the die having an outer diameter sized to increase the inner diameter of the end of the first tubular member to the nominal inner diameter of the end of the second tubular member; then (d) heat treating the end of the pipe to at a temperature less than a lowest transformation temperature of the pipe; then (c) machining a weld groove on the end of the pipe; then (d) abutting the ends of the pipe and connector and welding the ends together at the weld grooves.
 17. The method according to claim 16, wherein step (b) results in a reduction of the initial inner diameter in the range from 1% to 5%.
 18. The method according to claim 16, wherein steps (a) and (b) are performed by cold working processes.
 19. The method according to claim 16, wherein the inner diameter of the end of the first tubular member is increased in step (b) to within plus or minus 0.020 inch of the nominal inner diameter of the end of the second tubular member.
 20. The method according to claim 1, wherein the inner diameter of the end of the first tubular member is increased in step (b) to within plus or minus 0.010 inch of the nominal inner diameter of the end of the second tubular member. 